Macrophages and dendritic cells are constantly replenished throughout life and represent important components of the innate immune system. They were long thought to be produced within bone marrow in strict dependence on growth and differentiation factors. However, our new findings suggest that a special mechanism comes into play during times of systemic infection. When stimulated in culture with ligands for the Toll-like receptors (TLR) but no other factors, highly purified hematopoietic cells give rise to macrophages and dendritic cells. TLR have attracted considerable attention because of their display on mature effector cells and their ability to recognize bacterial/viral products. Our preliminary results indicate that TLR are expressed on stem cells and provide a primitive but highly effective mechanism for producing cells of the innate immune system. This two year project will exploit special knock-in and congenic strains of mice together with cell sorting and culture techniques to determine precisely which stem and progenitor cells express functional TLR. Cells from mice and humans will be exposed to viruses, anthrax and bacterial products to learn the range of response patterns. Subsequent experiments will reveal how individual progenitors proliferate and differentiate when exposed to these substances, as well as how macrophage production can occur at the expense of other blood cell types. The project will progressively move from simple experimental designs to complex whole animal studies. The aim is to learn if infections skew normal differentiation patterns so that the innate immune system can be quickly restored. TLR bearing cells are being implicated in autoimmunity and a wide range of other diseases relevant to this new line of investigation. These studies are certain to provide fundamental information, but could also suggest new means to augment the innate immune system. Lay Summary: Even plants and simple organisms have some defense against disease, and use what is referred to as the "innate immune system". Humans are also protected by an innate immune system, and although it is not as sophisticated as that achieved with immunization, it can quickly sense and respond to bacteria and viruses. This project is based on an exciting discovery about how the innate immune system may be restored during infection. The findings may also suggest a way that bones and stem cells in the bone marrow are harmed by infections. [unreadable] [unreadable] [unreadable] [unreadable]